The present invention relates to a telephone signal transmission device and, more particularly, to a digital speech CODEC capable of transmitting and receiving dial pulses in a network system which includes a telephone exchange.
In a digital telephone network, it has been customary for a telephone signal transmission device to transmit a dial signal at a rate of, for example, one bit for each 750 microseconds per channel in addition to a speech signal, as has been the case with a PCM24B system which transmits a 1.54 megabit/sec. signal, as shown in the specification set forth in CCITT G 704. Thus, the prerequisite for a telephone network to be constructed is that a dial signal be transmitted one bit for at least each 1.2 milliseconds or so in order to comply with a duty ratio of 33.5.+-.5 percent, which is the standard of dial distortion.
In a prior art telephone signal transmission device based on the PCM24B system, for example, a dial signal occupies only 1.3K bits per second, or 2 percent, of the total amount of information which may be transmitted on one channel, i.e. 64K bits per second. However, the situation is different when it comes to a band compression type CODEC which is extensively used with an office network and other private networks. Specifically, assuming that a dial signal is coded by such a CODEC at the lowest allowable bit rate of 800 bits per second, the proportion of the dial signal to the total information per channel amounts to 2.5 percent in a 32K bits per second ADPCM system and even to 33 percent in a 2.4K bits per second vocoder. This represents a dilemmatic situation that the lower the bit rate to which speech is coded by band compression technology, the greater the proportion of a dial signal to the total amount of information per channel is, resulting in various drawbacks such as degradation of the quality of sound.